Common sense about water use in the laboratory.

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1、（Tap water）
Tap water is usually of uncontrolled quality, may have seasonal variations such as level of suspended sediment depending on the source (municipal reservoir, river, well), may contain other chem-icals purposely added to drinking water (chlorine, fluoride), and is generally unsuitable for use in important experiments. Tap water is fine for washing glassware but should always be followed by a rinse with a higher-grade water (distilled, deionized, etc.).2、（Distilled Water ）
Distillation generally eliminates much of the inorganic con-tamination and particularly sediments present in tap water feedstock. It will also help reduce the level of some organic con-taminants in the water. Double distilling simply gives a slightly higher grade distilled water, but cannot eliminate either inorganic or organic contaminants.
Distilled water is often produced in large stills that serve an entire department, or building. The quality of the water is dependent on how well the equipment is maintained. A significant stir occurred within a large university’s biochemistry department when the first mention of a problem with the house distilled water was a memo that came out from the maintenance department that stated: “We would like to inform you that the repairs have been made to the still serving the department. There is no longer any radium in the water.” The next day, a follow-up memo was issued that stated:“Correction—there is no longer any sodium in the dis-tilled water.”
3、（Deionized Water ）
Deionized water can vary greatly in quality depending on the type and efficiency of the deionizing cartridges used. Ion exchange beds used in home systems, for instance, are used primarily to reduce the “hardness” of the water usually due to high levels of divalent cations such as magnesium and calcium. The resin bed consists of a cation exchanger, usually in the sodium form, which releases sodium into the water in exchange for removing the diva-lent ions. (Remember that when you attempt to reduce your sodium intake!) These beds therefore do not reduce the ionic content of the water but rather exchange one type of ion for another.
Laboratory deionizing cartridges are usually mixed-bed cartridges designed to eliminate both anions and cations from the water. This is accomplished by preparing the anion-exchange bed in the hydroxide (OH-) form and the cation-exchange resin in the acid (H+) form. Anions or cations in the water (including monovalent) are exchanged for OH-or H+, respectively, which combine to form neutral water. Any imbalance in the removal of the ions can result in a pH change of the water. Typically water from deion-izing beds is slightly acidic, often between pH 5.5 to 6.5.
The deionizing resins can themselves increase the organic con-taminant level in the water by leaching of resin contaminants, monomer, and so on, and should always be followed by a bed of activated carbon to eliminate the organics so introduced.
4、18MΩ(Reverse Osmosis/MilliQTM)
The highest grade of water available is generally referred to as 18MW water. This is because when the inorganic ions are completely removed, the ability of the water to conduct electric current decreases dramatically, giving a resistance of 18 MW.Com-mercial systems that produce this grade of water usually apply a multiple-step cleanup process including reverse osmosis, mixed-bed ion exchangers, carbon beds, and filter disks for particulates. Some may include filters that exclude microorganisms, resulting in a sterile water stream. High-grade 18 MW water tends to be fairly acidic—near pH 5. Necessary pH adjustments of dilute buffer solutions prepared using 18 MW water could cause discrep-ancies in the final ionic concentration of the buffer salts relative to buffers prepared using other water sources.
5、When Is 18MΩ Water Not 18MΩWater?
Suppose that your research requires 18 MW water, and you pur-chased the system that produces 500ml/min instead of the 2L/min version. If your research doesn’t require a constant flow of water, you can connect a 20L carboy to your system to store your pris-tine water. Bad Move.
18MW is not the most inert solvent; in practice, it is very aggres-sive. Water prefers the presence of some ions so as your 18 mW water enters the plastic carboy, it starts leaching anything it can out of the plastic,contaminating the quality of the water. The same thing happens if you try to store the water in glass. 18mW water loves to attack glass, leaching silicates and other ions from the con-tainer. If you need the highest purity water, it’s best not to store large quant